A conventional surface plasmon resonance sensor disclosed in Patent Literature 1 includes a prism, a metal film deposited on a bottom surface of the prism, and a probe fixed onto the metal film.
When light enters into the prism under a condition of total reflection, an evanescent wave develops on a surface opposite to the reflecting surface of the metal film, and is coupled with surface plasmon on the metal film. At this moment, a resonance occurs between the evanescent wave and the surface plasmon when a wave number of a component perpendicular to an interface of p-polarized light of the evanescent wave is in consonance with a wave number of the surface plasmon, and an intensity of electric field on the surface of the metal film increases. This results in absorption of the light, which reduces an intensity of reflected light leaving the prism in spite of the condition of total reflection.
The wave number of the surface plasmon depends upon a dielectric constant on the surface of the metal film and a refractive index of a medium contacting the metal. Therefore, changes occur in a wavelength (i.e., resonant wavelength) of the incident light and an incident angle (i.e., a resonance angle) that are the condition of resonance when an object substance is coupled with a probe provided on the metal film.
It is thus possible to analyze the object substance according to a change in relation between the resonant wavelength and reflectivity or a change in relation between the resonance angle and the reflectivity due to the presence or absence of the object substance.
A conventional localized plasmon resonance sensor disclosed in Patent Literature 2 includes plural metal particles arranged on a substrate at constant intervals. When light is radiated to these metal particles and causes a resonant vibration of free electrons in the metal particles with a vibrating frequency of electric field of the light, plasmon excitation occurs around the surfaces of the metal particles. The condition of localized plasmon resonance in this state is determined by the size of the metal particles and a dielectric constant around the particles. In a resonant frequency of the localized plasmon resonance, there emerges a peak of light absorption.
When an antibody coupled specifically with an antigen probe fixed to the metal particles is introduced to the sensor, a dielectric constant of the surfaces of the metal particles changes due to the coupling of the antibody with the antigen probe, hence changing the condition of the localized plasmon resonance. Accordingly, a reaction of the antibody, an object substance with the antigen probe can be sensed by detecting a change of an optical response of the metal particles. The optical response includes fluorescence, Raman scattering, and harmonic luminescence.
The conventional surface plasmon resonance sensor and the conventional localized plasmon resonance sensor exhibit only small changes in the reflectivity under the resonance condition, thus being prevented from having a high sensitivity.